1. Field of the Invention
The present invention relates generally to a detachable coupling for transmitting mechanical rotation from a driving shaft to a driven shaft which are axially aligned with one another.
2. Description of the Related Art
In technological apparatuses, it is often necessary to transmit a rotational motion without play from a drive which is installed in an apparatus to a pluggable subassembly. The engagement of the coupling and the detachment of the coupling should occur automatically without additional operational efforts.
In machines, it is often necessary for maintenance and operational purposes to be able to open the machines and to separate the parts of the machines. Detachable transmission elements are required as a standard for transmitting signals as well as for transmitting electrical and mechanical power. The couplings are required to be able to compensate for minor displacements of the subassemblies in the base apparatus.
An example of such a subassembly is a fixing station of electrophotographic singlesheet printer with a high printer output. In the fixing station, the paper runs between heated rollers. This fixes the toner on the paper. The fixing station must be easily removable by the user so that, for example, a paper jam can be cleared. The drive motor is advantageously built fixed in the housing of the printer since a step-down gear system must be connected between the motor and the fixing station due to the low rotational speed of the fixing station rollers.
A known apparatus which meets the requirement of easily coupling and detaching is a dog clutch, or coupling, as shown in FIG. 1. The functional principles of the coupling are explained here and after. A shaft end 2 of an element which is to be driven (such as a shaft in the fixing station) protrudes from a fixing station and is provided with a catch pin 2a. As the fixing station approaches the dog clutch 4 in an axial direction, the catch pin 2a can have an arbitrary angular position to a catch slot 4a. If the catch pin 2a does not align with the catch slot 4a when the shaft end 2 is advanced axially, it meets a helical surface 4b, which compresses the spring 6 which bears against an end 1b of the drive shaft and pushes the dog clutch over the drive shaft 1. When the drive rotates the sleeve 3, the catch pin 2a slides on the helical surface and after a partial rotation springs into the illustrated catch position. Another catch pin 1a which slides into a catch slot 3a of the dog clutch when the sleeve 3 is pushed along the drive shaft 1, transmits the rotational motion from a universal joint 5 which compensates for errors in alignment between the sleeve 3 and the shaft end 2. However, a problem in this arrangement is that both the catch pins 2a and 1a and the universal joint 5 must have some play to provided axial mobility. Given fluctuations in the drive torque, this play can lead to fluctuations in the rotational motion and, thus, to distortions in the print image produced by the fixing station.
Uniform rotational velocity is required for the fixing station because long papers are simultaneously positioned in the fixing station and in the transfer printing apparatus. Since the paper is clamped between the fixing rollers much more strongly in the fixing station than it is held in the transfer printing apparatus, any fluctuations in the rotational speed in the fixing station will cause the above-mentioned distortions in the print image.
To reduce torsional play, the known apparatuses use wedge-shaped grooves to improve performance, the wedge-shaped grooves being made free of play by the application of forces which act axially thereon. However, the problem is that an axial resilience is necessary which must also be able to transmit torque. A spring of this type which has sufficient torsional rigidity is not able to provide a sufficient length compensation. The length compensation is necessary, on one hand, to prevent destruction of the machine in case of an inadvertent meeting of the wedge-shaped groove teeth and, on the other hand, to avoid the high costs of providing a narrow position tolerance of the components in the axial direction. A further problem is presented by the application of axial forces which also require a corresponding stopping means.
A further known type of construction for couplings as described above uses a freewheel with a plug-in hub which acts at one side. However, an essential problem is that the torque transmission, which is effective only in one direction of rotation, does not offer sufficient synchronization in low-friction systems and upon the occurrence of varying torque superpositions, such as during run-in moments or stresses in the drive train.
In a coupling means as previously known, a uniform transmission of the rotational motion is not always ensured. In mechanical power couplers, the problems lie in the torsional play that arises due to joints that compensate for axial displacement and/or for length compensating grooves provided for axial resilience. This has a particularly disadvantageous effect, especially in connection with stepped motor drives and in the case of high synchronization demands such as in transfer printing processes in the above-described electrophotographic printer.